1. Field of the Invention
The invention relates to syntactic foams and more particularly relates to curing syntactic foam made with polyepoxide resin binders.
2. Brief Description of the Prior Art
The literature is replete with descriptions of prior art syntactic foams made by curing mixtures of microspheres with curable polyepoxide resin binders; see for example the description given in the U.S. Pat. No. 3,849,350.
The prior art techniques of curing the polyepoxide resin binders in syntactic foam forming compositions have been, in general, satisfactory when the article to be molded is relatively low in density i.e.; having a density below about 35 lbs./cubic foot. Higher density mouldings are not always satisfactory. The difficulty arises because the curing process is a chemical reaction, exothermic in nature. The general heat of reaction is desirable to sustain the curing reaction to completion. However, the syntactic foam being formed is a thermal insulator and as the exotherm continues, thermal energy is trapped within the body of the moulding. In high density mouldings the heat build-up on the interior of the moulding can exceed temperatures which are degradative to the foam composition, i.e.; above about 400.degree. F. When this happens, the resin binder of the moulding will degrade, split, char, become brittle, discolor and weaken.
By the method of the present invention, relatively dense mouldings may be made from syntactic foam forming compositions having a curable polyepoxide resin binder ingredient without exceeding internal temperature limits which would be degradative of the cured polyepoxide resin binder. The mouldings are particularly useful as buoyancy devices in marine applications.